1. Field of the Invention
The present invention is directed to the field of polymerization of ethylene to prepare low to medium density bimodal polyethylene polymers.
2. Background Art
The greatest part of commercial polyethylene production occurs in gas phase and slurry loop processes. Gas phase processes are in widespread use, and are typified by processes such as that disclosed in U.S. Pat. No. 4,003,712. In such processes, the temperature and pressure are selected such that ethylene and comonomers are introduced at the lower end of a vertical reactor in gaseous form, and, in addition to serving as the monomer source, also serve to fluidize the bed of polyethylene polymer particles as they rise to the bulbous head of the reactor. Polyethylene particle product falls along the internal walls and is harvested from the bottom section of the reactor. Such reactors have several noted disadvantages: gas phase reactors are technologically complex and require process conditions to be controlled very precisely in order to maintain normal operation; fouling and electrostatic phenomena limit the range of products that can be produced; bimodal resins are difficult to prepare and increase technological complexity, often requiring multiple catalysts; and the cascading of such reactors is very difficult if not practically impossible.
Slurry loop reactors are also in widespread use. Loop reactors may be of horizontal or vertical configuration, and are multiple loop, tube-within-tube reactors, with coolant flow directed through the space between the concentric tubes. Polymerization takes place in a liquid carrier solvent, generally isobutane, the reactants and polymer being driven around the loop by a large capacity pump. Loop reactors are operated at nominally 600 psi (41.4 bar) to ensure the reaction system operates liquid full, i.e. above the vapor pressure of the solvent mixture, so that proper slurry recirculation and heat transfer is maintained. Product takeoff may be continuous, but is generally by means of multiple settling legs where polymer particles partially sediment and are removed at intervals, at a higher solids content than the solids content of the reactor itself. A typical slurry reactor is illustrated in U.S. Pat. No. 4,068,054.
It has been proposed to cascade slurry loop reactors to produce bimodal resins, but generally only when the first reactor is operated hydrogen-free. In U.S. Pat. No. 6,221,982, it has been proposed to remove hydrogen from a first cascaded slurry reactor by employing a separate catalyst in the reactor effluent which causes hydrogenation of unreacted ethylene, a process wasteful of both hydrogen and ethylene, and which requires the addition of an expensive metallocene catalyst which does not contribute to polymer production. In U.S. Pat. No.6,225,421, it has been proposed to remove hydrogen from the effluent of a first cascaded slurry reactor, but no details of any removal method are given. By hydrogen is meant diatomic hydrogen.
Although slurry loop reactors are generally more robust systems than gas phase reactors, they are nevertheless prone to foul during the production of high melt index and lower density resins. Lower molecular weight polyethylene oligomers and waxes tend to be soluble in the continuous phase, and addition of increasing amounts of higher olefin comonomers such as 1-butene and 1-hexene to produce low to medium density resins render the polyethylene copolymer product increasingly soluble. Below a density of about 0.95, the reactor mixture becomes increasingly xe2x80x9csoupy.xe2x80x9d In order to operate under such conditions, the coolant temperature has to be raised to prevent fouling, which necessarily requires production rates to be reduced, thus negatively impacting the process operating economics.
Heavy solvent stirred tank series reactor technology has been commercially employed to produce bimodal polyethylene copolymers for over two decades. Stirred tank reactors are less technically complex than gas phase or slurry loop polyethylene reactors. Generally, stirred tank reactors are designed with a nominal aspect ratio (height:diameter) of 4. Slurry reactors are usually jacketed vessels, which typically employ steam for heating up the reactor system for startup. Heavy solvent stirred tank polyethylene reactors typically operate at low to moderate pressures (50 to 350 psi; 3.4 to 24 bar), at a temperature similar to both gas phase and slurry loop reactors (140xc2x0 F. to 190xc2x0 F. (60-88xc2x0 C.)). Like loop reactors, the reaction mixture is a solid/liquid slurry consisting primarily of polymer and a liquid carrier solvent, generally hexane. The homogeneity of the slurry is maintained by mixing, generally with a conventional agitation and baffling system, such as shaft driven disk turbine agitators with reactor wall baffles. Also like gas phase and loop reactors, the heat of polymerization in a stirred tank reactor is removed, i.e. reaction temperature is controlled, using sensible heat transfer, typically with cooling water in the reactor jacket, with internal coolers, with external coolers, or any combination thereof.
Heavy solvent stirred tank polyethylene reactors are generally simpler and more robust than either gas phase or loop reactor systems. However, like loop reactors, stirred tank reactors are prone to foul during the production of high melt index (low molecular weight) and lower density (high comonomer composition) resins, a problem that is exacerbated by the higher degree of miscibility that low molecular weight and lower density polyethylene have in the heavy hexane solvent. Thus, bimodal polyethylene copolymers with densities below nominally 0.930 cannot be practically manufactured with heavy solvent stirred tank reactors in series processes.
Heavy solvent stirred tank reactor processes are also generally more expensive to build and operate than gas phase or loop reactor processes due to the difficulty in separating the heavier solvent from the product polyethylene, which typically requires centrifuges and complex mechanical drying equipment. Such equipment is both more expensive and less reliable than the polymer drying systems employed in gas phase and loop reactor manufacturing systems.
Boiling pool reactors have been proposed long ago, for example in several patents assigned to the Koppers Co., i.e. U.S. Pat. Nos. 2,885,389; 2,918,460; 3,126,460; and GB Patent 826,562. In the processes disclosed, elongated vertical reactors are operated in a slurry mode at a pressure and temperature such that the hydrocarbon continuous phase boils, and is subsequently condensed and returned to the reactor. While a number of solvents including propane are mentioned, all the polymerizations involve use of pentane, hexane, or cyclopentane. No copolymerization is disclosed, and only relatively high molecular weight products are produced. While boiling pool technology was disclosed as early as 1955, the use of these reactors to prepare polyethylene commercially is unknown, although polypropylene is commercially produced in such processes. No boiling pool series reactor processes or bimodal polyethylene manufacturing processes are disclosed.
It would be desirable to provide a flexible process for the preparation of polyethylene copolymers with a bimodal molecular weight distribution which is more operationally robust than gas phase or slurry loop processes, provides competitive installed costs and operating economics, and in which great flexibility in both hydrogen and comonomer usage is possible. It is also desirable to provide a process where bimodal polyethylene polymers may be prepared in a slurry reactor at densities lower than those achievable with existing heavy solvent series reactor technology.
The present invention pertains to a two stage polyethylene copolymerization process involving two series connected adiabatic boiling pool reactors employing light solvent. By means of the inventive process, it has been surprisingly discovered that low to medium density bimodal polyethylene copolymers may be readily produced without resort to complex and expensive product drying processes.